This invention pertains to method and apparatus for the manufacture of hydrophone streamers useful for seismic exploration over water. More particularly the invention relates to method and apparatus for filling such streamers with a predetermined amount of oil to achieve the desired degree of streamer buoyancy, which may be positive, negative, or neutral.
A seismic streamer comprises a cable harness inside an oil filled sheath. The sheath is a flexible plastic tube. The harness includes a plurality of hydrophones and associated electrical conductors, strain rope, spacers, and bulkheads. Method and apparatus for the manufacture of a hydrophone streamer are disclosed in U.S. Pat. No. 3,885,286 to A. C. Hill, assigned to the same assignee as the present application, the disclosure of that patent being incorporated herein by reference.
According to the disclosure of the Hill patent prior to assembly with the sheath, the harness is wound on a reel inside a pressure vessel containing oil and air. One end of the harness is fed through a spigot at the side of the vessel below the oil level and connected to a free piston which is inserted into one end of the sheath. The sheath is attached to the spigot at the side of the vessel below the oil-air interface.
The air in the tank is presurized placing the oil under pressure, and the harness and oil are fed to the sheath through the spigot. The pressurized oil forces the piston to move along the several hundred feet of sheath, pulling the harness into the sheath at a rate regulated by unwinding of the reel. The reel is actuated by a hydraulic motor.
The piston is provided with a lubricator at its leading end, a pint or so of oil being placed in the sheath before it is attached to the spigot. The sheath is initially expanded by admitting air under pressure through a check valve at the other at the other end of the sheath from the end attached to the spigot. A pressure relief valve maintains the desired air pressure in the sheath.
Since the sheath, or jacket as it is now called, expands during insertion of the harness, the precise internal volume of the jacket is variable. This results in the amount of oil in the streamer being uncertain, even though the jacket is "filled up" with oil in the process of inserting the harness, in order to get the harness all the way into the jacket and in order to eliminate air bubbles from the streamer.
It is desirable to end up with only a predetermined amount of oil and no air in the streamer, after the harness has been inserted. That is the objective of the invention.